Project Summary Heart disease remains the leading cause of death in the United States, accounting for ~25% of total deaths. While the underlying etiologies often involve specific insults such as myocardial infarction or ion channel mutations, clinical outcomes for patients vary based on the response of the heart as a whole, a fact that is increasingly recognized in clinical settings. The important connection between the topology of the heart and its dynamic behavior, and how both are altered through myocardial injury, has informed a number of recent therapeutic advances, such as cardiac resynchronization therapy in the management of conduction block and heart failure and targeted catheter ablation for refractory arrhythmias. While promising, our understanding of, and thus ability to manipulate, cardiac behavior as a function of topology is still relatively simplistic. For instance, computational models have shown that system behavior is extremely sensitive to the position of conduction blocks such as scar tissue, but these results require validation in biological systems. This has so far proved infeasible due to an inability to manipulate tissues with the same resolution and throughput as in silico models. Continued progress in arrhythmia management requires better characterization of heart behavior as a function of topology and the ability to develop novel spatiotemporal control schemes in biological systems. The first aim of this study is thus to develop a novel in vitro system for the pursuit of these goals. Induced pluripotent stem cell lines are being developed that express light-sensitive ion channels and pumps. These cells will be differentiated into cardiomyocytes for the formation of cardiac tissue constructs. A projector has been integrated into an optical mapping system, allowing for simultaneous imaging and stimulation of these tissues. The second aim is to characterize tissue arrhythmogenicity as a function of tissue topology by analyzing conduction blocks created through the application of spatially heterogeneous light patterns to cardiac tissue expressing light-sensitive inhibitory ion pumps. The third aim is to evaluate novel spatiotemporal control algorithms for arrhythmia control by stimulating cardiac tissue expressing light-sensitive excitatory ion channels with time-varying, spatially heterogeneous light patterns. This approach will lead to improvements in current strategies for the acute termination of arrhythmias, and for the maintenance of normal function in pro- arrhythmic conditions.